Timing of diapause induction outside the natural geographical range of a species: An outdoor experiment with the bean bug Riptortus clavatus

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[3351] ,, E-mail: Saulich@AS1061.spb.edu. The phytophagous bug Riptortus clavatus (Heteroptera: Alydidae) produces 2 or 3 generations per year in Central Japan and overwinters at the adult stage. In the Kyoto (35°00?N, 135°45?E) population, we studied (1) the effects of day-length on the nymphal and preoviposition periods under constant photoperiod at 20.5°C, (2) photoperiodic induction of adult diapause at 20.5°C and under a combination of constant photoperiod and natural daily rhythm of temperature in the forest-steppe zone of Russia (50°38?N, 35°58?E). Then, we examined (3) the timing of diapause induction under quasi-natural conditions in the same region, far outside the species’ natural range. At 20.5°C, the nymphal period in both males and females was significantly longer in the regimes with shorter photophases than under those with longer photophases. The preoviposition period in females was significantly longer under the near-critical long-day regime L14:D10 than under typical long-day regimes (L15:D9, L16:D8 and L17:D7). The critical day-length for diapause induction was shorter under conditions of natural daily rhythm of temperature than those reported at constant 20, 25 and 30°C. Under quasi-natural conditions in the forest-steppe zone, R clavatus entered diapause in September, much later than the local populations of true bugs studied to date and this finding is well explained by the characteristics of the phtoperiodic response of this species. These results illustrate the proposition that when a species with a photoperiodically induced diapause is introduced into new environmental conditions far outside its natural geographical range, day-length and temperature remain the leading factors in the control of seasonal development and diapause induction. The results will be discussed in comparison with the findings in other heteropterans in the same region and with the records of seasonal development of R. clavatus in Kyoto.

Index terms: Heteroptera, photoperiodism, seasonal adaptations, life cycle, voltinism

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